Conventionally, in an image forming apparatus such as a printer and a copying machine, when an image is outputted based on inputted image data of multi-step gradation, the gradation of the inputted image data is corrected so as to precisely reproduce the gradation thereof according to a property of a device adopted for an input or output system.
Such a technique for correcting gradation is disclosed in publications such as {circle around (1)} Japanese Laid-Open Patent Publication No. 173942/1998 (Tokukaihei 10-173942, published on Jun. 26, 1998) and {circle around (2)} Japanese Laid-Open Patent Publication No. 278347/1998 (Tokukaihei 10-278347, published on Oct. 20, 1998).
In technique {circle around (1)}, firstly, a reference patch (gradation pattern) is formed on a light sensitive element, a target concentration value of the reference patch is stored and is directly read by an image reading means, and then, correction data is generated for correcting a property of concentration gradation on a formed image, according to a difference between the stored target concentration value and an actually read concentration value. Hence, the gradation can be corrected in a short time with high accuracy.
In technique {circle around (2)}, based on pattern information, in which test patterns (gradation pattern) with continuous gradation are formed so as to be adjacent to one another, a test pattern is formed in which different gradations are aligned in a two-dimensional manner, and the image forming conditions are controlled by reading the test pattern. Thus, it is possible to precisely detect a property of an image forming means, so that correction can be made to preferably form an image.
Moreover, when forming an image of the gradation pattern as a chart on a sheet in order to make the above gradation correction, if an electrostatic latent image corresponding to the gradation pattern is formed on a light sensitive element, a part such as an end of the gradation pattern rapidly changes its electrostatic potential. Such a part has a concentration higher than the original one. This phenomenon is generally called an edge effect. Technique such as {circle around (3)} Japanese Laid-Open Patent Publication No. 160075/1995 (Tokukaihei 7-160075, published on Jun. 23, 1995) is disclosed for preventing the edge effect.
In technique {circle around (3)}, a quantity of reflected light is detected by a toner adhesion part and the other part on the light sensitive element, the detection result is subjected to time quadrature with a first degree or more, a maximum value of the time quadrature result is detected, and the concentration of the image is controlled according to a ratio of a maximum value of the detected adhesion part and a maximum value of the other part. Therefore, the concentration control is not affected by the edge effect.
Moreover, regarding an image forming apparatus using an electrophotographic process, a technique is available in which a base color is detected under a sheet having an image thereon, and the concentration of a formed image is adjusted according to the base color. With this arrangement, a desirable image can be formed according to the base color of the sheet. Such a technique is disclosed in {circle around (4)} Japanese Laid-Open Patent Publication No. 230870/1990 (Tokukaihei 2-230870, published on Sep. 13, 1990, corresponding to U.S. Pat. No. 5,657,395) and {circle around (5)} Japanese Laid-Open Patent Publication No. 145598/1998 (Tokukaihei 10-145598, published on May 29, 1998) and others.
In technique {circle around (4)}, before a final scan for reading a document, that is performed for outputting an image signal, a maximum concentration and a base concentration of a predetermined area on the document are detected so as to compute concentration coefficient data for correcting image data and base data which corresponds to the base concentration. Further, in the final scan, a base removing means reduces the image data according to the base data. A concentration correcting/operating means performs an operation for increasing an output from the base removing means according to the concentration coefficient data, and outputs a resulting correction image data. This arrangement makes it possible to form a clear image with high contrast regardless of a concentration of the base.
Next, in technique {circle around (5)}, a plurality of reference patches are formed with different concentrations, and a concentration on each of the reference patches and the base are measured. A reference correction of the base is computed according to a measurement value and a predetermined target value of the base concentration, and regarding each of the reference patches, a concentration measurement value or a predetermined target value of concentration is corrected according to correction quantity, which is not more than the computed reference base correction and decreases as the concentration of the reference patch is higher. With this arrangement, it is possible to reduce errors resulted from a various kinds of sheets on a highlighted part and a various kinds of concentration measurement devices, while preventing excessive correction on a high-concentration part.
Additionally, when the gradation pattern is read by an image reading means, interference may appear between spacial frequencies of a document having the gradation pattern thereon and a CCD (image reading means), resulting in moire, etc. {circle around (6)} Japanese Examined Patent Publication No. 95240/1994 (Tokukohei 6-95240, published on Nov. 24, 1994) discloses a technique for preventing such interference.
In technique {circle around (6)}, when a test chart is read by the image reading means, an optical path length of an image-forming optical system in the image reading means is changed from an optical path length used for reading an ordinary document; thus, a spacial frequency of the image reading means is changed so as to prevent interference between spacial frequencies of the image reading means and an image outputting means. Therefore, it is possible to precisely read a test chart by the image reading means and to precisely adjust a property. Besides, the property adjustment does not depend upon a spacial frequency of the image output means, so that a property can be automatically adjusted in response to a variety of image outputting means having different spacial frequencies.
In addition, a relationship between a concentration input value and a concentration output value is generally referred to as γ property. Namely, the γ property is a parameter indicating a scale of gradation. The γ property is arbitrarily corrected so as to adjust the gradation of a formed image in a suitable manner. Technique {circle around (1)} and {circle around (7)} Japanese Patent Publication No. 2643951 (Patent No. 2643951, published on Aug. 25, 1997) relates to a technique for correcting the γ property.
In technique {circle around (1)}, as described above, correction data, which corrects a property of concentration gradation (γ property) on a formed image, is generated according to a difference between the stored target concentration value and an actually read concentration value.
In technique {circle around (7)}, correction data of the image reading means is generated based on image data of a reference chart that is read by an image reading means. An image is corrected by an output means according to reference data for correcting the output means and is read by the image reading means which is corrected by the generated correction data. Correction data of the outputting means is generated based on the read image data.
With this arrangement, the image reading means and the output means can be separately corrected, and it is possible to precisely correct a change in properties of means. Moreover, at this point, the image reading means for correcting a property is used for generating correction data of the output means. Hence, the output means can be corrected with a simple construction without the necessity for a special input device.
Incidentally, a main scanning direction is more susceptible to the edge effect than a sub-scanning direction. Further, regarding an image forming apparatus using an electrophotographic process, when an electrostatic latent image is developed so as to correspond to a gradation pattern, a concentration of toner is determined based on a concentration of a representative place. The toner concentration decides if the toner should be replenished or not. For this reason, if gradation patterns with high concentration successively appear in the sub-scanning direction, responsivity in replenishing the toner may be deteriorated.
Therefore, without considering the edge effect and responsivity in replenishing the toner to an electrostatic latent image, it is not possible to form a gradation pattern having suitable gradation. Also, it is not possible to read a concentration of the gradation pattern by the image reading means, resulting in improper adjustment of the image forming apparatus.
However, techniques {circle around (1)} and {circle around (2)} do not discuss a reduction in accuracy of reading, that occurs on the gradation pattern due to the edge effect. Further, technique {circle around (3)} can correct uneven concentrations that appear on the gradation pattern due to the edge effect. However, the responsivity of a developing bias, etc. is less likely to change in response to an edge of the gradation pattern. Consequently, the gradation pattern is less likely to have a desired concentration, so that a suitable gradation pattern cannot be achieved.
Meanwhile, in techniques {circle around (4)} and {circle around (5)} relating to image correction based on a base color of a sheet having an image formed thereon, image data is read and is simply corrected according to base data of the sheet. Here, a zero level regarding a reading value of the image reading means is determined by reading a reference whiteboard. Regarding the sheet having a gradation pattern thereon, when a degree of white is higher than the reading value of the reference whiteboard, the base and the gradation pattern cannot achieve a suitable contrast for gradation correction.
Therefore, when the gradation pattern is read on the sheet, a readout value of the gradation pattern does not correspond to a white level of the base, so that the image reading means cannot obtain a suitable reading value, resulting in insufficient gradation correction. Namely, it is not possible to effectively use the formed gradation pattern.
Besides, in technique {circle around (6)} for preventing interference between spacial frequencies of a document and the image reading means, an optical path length of the image-forming optical system is changed to prevent interference. Namely, in technique {circle around (6)}, a mechanical means is necessary for changing the optical length, and a driving means is further required for improving accuracy of reading a gradation pattern. For this reason, the construction of the apparatus becomes more complicated.
In techniques {circle around (1)} and {circle around (7)}, simply, the gradation pattern is read and a curve of a reference γ property is changed accordingly. Moreover, when correcting gradation of an image, a visual gradation pattern chart may be used in addition to a reading gradation pattern chart which is read by the image reading means. The visual gradation pattern chart prevents an inter-gradation concentration from being recognized as a different concentration. However, the foregoing techniques do not discuss the setting of the number of the reading gradation patterns and the number of the visual gradation patterns.